Digital chrominance signal demodulation apparatus

ABSTRACT

Different from typical signal processing which employs a feedback control, by adopting a demodulating circuit employing AFC, which is not affected by a comb filter, the response characteristic against jitter is improved and a down converted chrominance signal can be demodulated with a good accuracy. Therefore, the noise rejection effect by a comb filter is improved, the detecting accuracy of the residual phase error is also improved, and the S/N ratio of the phase is improved by combining feedforward APC compensation with a velocity error, and this results in a much improved picture quality.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to chrominance signal processing in a commercialvideo cassette recorder (VCR) and handles demodulation from a downconverted chrominance signal into a color difference signal by digitalsignal processing.

2. Description of the Prior Art

Recently, various systems have been proposed to improve the performanceof a VCR by introducing digital signal processing.

In the case of a chrominance signal, although a signal format isstipulated as a signal standard, there is no particular standard onsignal processing. Therefore, a circuit configuration suitable fordigital processing can be adopted with some flexibility.

A digital chrominance signal processing apparatus proposed for a VHSsystem is explained below.

For example, according to a Japanese patent publication no. 4-4799 (Feb.12, 1985, "a chrominance signal processing apparatus") which isincorporated herein by reference, a reproduced down converted carrierchrominance signal is demodulated at a demodulator after beingA/D-converted.

Phase error is detected from a color difference signal (R-Y) of the twodemodulated color difference signals at a burst gate circuit. The errorsignal is D/A-converted, a clock signal corresponding to input jitterfrequency is generated by controlling an analog variable frequencyoscillator, and by applying the clock signal to the A/D converter andthe demodulator mentioned above, a feedback APC (Automatic PhaseControl) loop is completed.

But, in this feedback APC loop, if a quick response occurs, one signalprocessing becomes unstable and, moreover, when a cross-talk componentexists in the reproduced signal such as in a long play mode, a combfilter is necessary in the loop. Thus, it is easy for the signalprocessing to become unstable.

Therefore, in the publication mentioned above, in order to improve uponthe deficiency, an improvement of the response characteristic isattempted which includes adopting a feedforward APC in the output of thefeedback APC. In particular, a phase error at a present line iscalculated using burst signals and the color difference signals (R-Y)and (B-Y), and a vector operation to restore it to the normaldemodulation axis is performed by an operation circuit.

A chrominance signal processing apparatus, as described in IEEE Trans.on CE, vol. 36, no. 3, Aug. 1990, p. 560-566, which is incorporatedherein by reference, has basically the same configuration as theabove-mentioned publication; however, it is different in that thefeedback loop is digitally processed.

In the prior art, as a feedback APC is adopted for color demodulation, ahigh frequency jitter component cannot be removed. Moreover, when a combfilter exists in the loop, the response characteristic against jitterbecomes worse.

Thus, if a color difference signal, in which the jitter component is notsufficiently removed, is passed through a comb filter, an intrinsicfilter characteristic cannot be obtained and the ability to reject across-talk component and a noise component decreases.

As a result, because the detecting accuracy of a phase error from aburst signal decreases, the jitter suppressing effect becomesinsufficient, even in the feedforward APC adopted in order to improvethe response characteristic.

SUMMARY OF THE INVENTION

The present invention relates to a digital chrominance signaldemodulation apparatus which includes:

carrier generator means for outputting two orthogonal carriers based ona first input signal and a second input signal;

demodulator means for demodulating a down converted carrier chrominancesignal, which is a reproduced signal of a video cassette recorder, bythe outputs of said carrier generator means and for outputting a firstcolor difference signal an a second color difference signal;

first comb filter means and second comb filter means for comb filteringsaid first color difference signal and said second color differencesignal, respectively and for producing respective outputs;

first delay means and second delay means for delaying the outputs ofsaid first comb filter means and said second comb filter means by apredetermined period, respectively;

phase error detector means for detecting a residual phase error in theoutput of said first comb filter means and the output of said secondcomb filter means during a Bate pulse and producing an output; and

phase error compensator means for compensating the residual phase errorin the outputs of said first delay circuit and said second delay circuitby employing the output of said phase error detector.

A down converted carrier chrominance signal is demodulated with twoorthogonal carriers generated in the carrier generator.

The two demodulated color difference signals are applied to the phaseerror detector and to delay circuits through respective comb filters.The two color difference signals delayed by a specified period throughthe delay circuits are compensated at the phase error compensatoraccording to a phase error signal from the phase error detector. Thus,high quality color difference signals with no jitter component can beobtained at the output of the phase error compensator.

According to the present invention, the response characteristic isimproved as compared with an APC type, because demodulation is done onlyin an AFC (Automatic Frequency Control) circuit which has no feedbackloop.

Therefore, comb filters can remove a cross-talk component and a noisecomponent as an intrinsic characteristic and the detecting accuracy ofthe residual phase error is improved. In addition, by adopting a phaseerror compensation with a high performance feedforward type, a higherjitter suppression effect is obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of digital chrominance signal demodulationapparatus in accordance with an exemplary embodiment of the presentinvention.

FIG. 2 is a block diagram of a carrier generator 11 in FIG. 1.

FIGS. 3a and 3b illustrates waveforms of an AFC 20 and a phase shifter21 in FIG. 2.

FIGS. 4a and 4b are vector diagrams showing a relation betweendemodulation axes of an NTSC signal and a signal having a phasedifference of θ.

FIGS. 5(a)-5(c) illustrate waveforms showing a time base error and avelocity error in accordance with an exemplary embodiment of the presentinvention.

FIG. 6 is a block diagram of a phase error detector 16 in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary embodiment of the present invention is illustrated by FIGS.1 through 6. FIG. 1 is a block diagram of an exemplary embodiment of adigital chrominance signal demodulation apparatus.

A down converted carrier chrominance signal is applied to the inputterminal la and two orthogonal carriers generated in the carriergenerator 11 are input to the demodulator 10. In the demodulator 10,demodulation by synchronous detection is performed and undesiredcomponents of the signal generated at the synchronous detection areremoved. Two color difference signals Er and Eb are obtained at theoutputs of the demodulator 10.

Here, the carrier generated in the carrier generator 11 is synchronizedwith a frequency which is 40 times the horizontal sync (HS) frequency. Ablock diagram of the carrier generator 11 is shown in FIG. 2.

Referring to FIG. 2, a sawtooth wave ωc(n) (where n is a naturalnumber), having 40 periods during each period of a HS pulse, isgenerated at the automatic frequency control circuit AFC 20. The HS isapplied to the terminal 1b and the sawtooth wave is synchronized withthe HS signal.

Next, the phase of the sawtooth wave ωc(n) is shifted by 90 degrees(lead or lag) during every period of a horizontal sync pulse (HS)according to the polarity of a rotary signal (RS) applied to the inputterminal 1c of the phase shifter 21.

Carriers, which are orthogonal to each other and expressed as sin{ωc(n)}and cos{ωc(n)}, are output from a sin-Table 22 and cos-Table 23,respectively.

FIGS. 3a and 3b show waveforms illustrating the performance of AFC 20and phase shifter 21 in FIG. 2. FIG. 3(a) is a waveform of the outputωc(n) of the AFC 20. Assuming its dynamic range is 2^(n), it has 40cycles of sawtooth waves during a period t_(H), where t_(H) ishorizontal sync period.

FIG. 3(b) shows that the phase of the output of the phase shifter 21 isshifted by 90 degrees every period t_(H). (Here, it is illustrated thatthe phase is leading.)

As the dynamic range is 2^(n), the phase information of 90 degrees isexpressed by 2.sup.(n-2). Therefore, in order to lead the phase by 90degrees every period t_(H), the value 2.sup.(n-2) is added one afteranother to the value ωc(n) every period t_(H). Thus, the phase leads by360 degrees every four periods 4t_(H). But, as the carrier generator 11includes AFC 20, the generated carrier can respond only to frequency.

Therefore, when the down converted carrier chrominance signal ismodulated by a signal with frequency ωc(n) and phase θ, the colordifference signals Er and Eb passed through the comb filters 12 and 13,respectively, are demodulated such that the demodulation axis isdeviated by an initial phase θ.

This is shown in FIGS. 4(a) and 4(b). FIG. 4(a) is a vector diagram ofthe case in which the down converted carrier chrominance signal isdemodulated with a normal demodulation axis. Er, Eb, and A expressamplitudes of a red color difference signal (R-Y), a blue colordifference signal (B-Y), and a burst signal, respectively.

FIG. 4(b) is a vector diagram of the case in which the down convertedcarrier chrominance signal is demodulated with a demodulation axis of anuncompensated initial phase θ.

Er and Eb are color difference signals (R-Y) and (B-Y), respectively,and A is a burst signal which has components Ar and Ab expressed by eq.(1) and eq. (2) on the axes (R-Y) and (B-Y), respectively.

    Ar=A·sinθ                                   eq. (1)

    Ab=-A·cosθ                                  eq. (2),

and the relationship between ER-Er and EB-Eb is

    ER=Eb·sinθ+Er·cosθ           eq. (3),

    EB=Eb·cosθ-Er·sinθ           eq. (4).

If some jitter component is left in the AFC 20 of the carrier generator11 without being completely removed, the phase error θ is not constantbut rather a function of time; in particular, it varies sinusoidally asshown in FIG. 5(a).

If the phase error θ(t) is detected only for particular values of t (or,put another way, at particular points along the x-axis of FIG. 5(a), astep-wise function is obtained. This is called "time base error" and isshown in FIG. 5(b).

However, if the error compensation is done only by a phase error θ(t_(H)) detected at the time t_(H) in FIG. 5(b), a phase error as shownin FIG. 5(c) is obtained at time t_(H+1). In other words, the gradientof θ(t) values along neighboring points t is generated. Thispseudo-sawtooth is called a "velocity error" and is shown in FIG. 5(c).

In order to improve the jitter characteristic, the phase error of thefirst degree holding characteristic should be compensated for everysampling during a line.

FIG. 6 shows a block diagram of a phase error detector 16, which detectsa velocity error and calculates a compensation data at every sampling.If color difference signals applied to the input terminals 6a and 6b arethe signals at the time t_(H+1) in FIG. 5(b), i.e. the signal of the(H+1)th line, the amplitudes of the burst signals are expressed by eq.(5) and eq. (6), respectively.

    Ar.sub.·H+1 =A.sub.H+1 ·sinθ.sub.H+1eq. (5)

    Ab.sub.·H+1 =-A.sub.H+1 ·cosθ.sub.H+1eq. (6)

These signals are averaged during the period of the burst gate pulse(BG), which is applied to the input terminal 1d, at the burst signalaveraging circuits 600 and 602, and are logarithmically transformed atthe log-Table 601 and 603, respectively, and then they are subtracted atthe first subtracter 604 in FIG. 6.

The output of the first subtracter 604 is expressed by eq. (7).

log(A_(H+1) ·sinθ_(H+1))"log(A_(H+1) ·cosθ_(H+1)) =log(tanθ_(H+1)) eq.(7)

This signal is exponentially transformed at the exp-Table 605. Next, itis transformed to its arc-tangent value at the arctan-Table 606. Thus,the time base error θ_(H+1) during the (H+1)th line is calculated.

At the holding circuit 607, a time base error θ for the previous line isheld and, a velocity error is calculated as (θ_(H+1) -θ_(H)) at thesecond subtracter 608. This value is divided by N at the 1/N circuit609, where N is a number of sampling data during one line. The output ofthe 1/N circuit 609 is integrated at the integrator 610 and its outputand the output of the holding circuit 607 are added to each other at theadder 611. At the output terminal 6c of the adder 611, a compensationdata θ(n), (wherein n is 0≦n≦N-1) at every sampling is obtained and itis expressed by eq. (8).

    ⊖(n)=θ.sub.H +(θ.sub.H+1 -θ.sub.H)·n/Neq. (8)

The outputs of the comb filters 12 and 13 are input to the delay circuit14 and 15, respectively which delay the outputs for a predeterminedperiod of time. This is because the object of compensation is theprevious color difference signal. To calculate a compensation data, fromthe phase error between two adjoining lines is used. For a delaycircuit, a line memory may be used.

The color difference signals Er and Eb, which are the outputs of thedelay circuits 14 and 15, and the compensation data θ(n), which is theoutput of the phase error detector 16, are used to calculate by way ofeq. (3) and eq. (4) at the phase error compensator 17, the colordifference signals ER and EB which are θ'(n)=0. This means that anoutput having residual jitters almost completely removed are obtained atthe output terminals 1e and 1f of the phase error compensator 17.

Thus, by adopting an AFC type in the carrier generator 11, it becomespossible to get a full performance of feedforward APC which cannot beachieved with a usual feedback APC type.

In the invention explained above, color difference signals always passthrough comb filters but another circuit configuration, in which thecomb filters are switched to on or off according to whether therecording is in a long play mode or in a standard mode, is alsopossible.

Although the explanation was given regarding an NTSC system, thisapparatus is adaptable to all signal processing employing a downconverting method such as PAL system.

An advantage of this invention is that by employing an AFC circuit witha better response characteristic as compared with feedback APC,chrominance signal demodulation with improved accuracy is obtained.Moreover, in this invention, the same response characteristic isrealized in a standard mode and in a long play mode, as they are notaffected by comb filters.

Furthermore, as a cross-talk component and a noise component arecompletely removed in a comb filter, the detecting accuracy of theresidual phase error (time base error) is also increased and it resultsin the velocity error being accurately calculated. This results insufficient compensation at every sampling and a substantially improvedchrominance signal quality.

The invention may be embodied in other specific form without departingfrom the spirit or essential characteristics thereof. The presentembodiment is therefore to be considered in all respects as illustrativeand not restrictive, the scope of the invention being indicated by theappended claims rather than by the foregoing description and all changeswhich come within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

What is claimed:
 1. A digital chrominance signal demodulation apparatuscomprising:carrier generator means for outputting two orthogonalcarriers based on a first input signal and a second input signal;demodulator means for demodulating a down converted carrier chrominancesignal, which is a reproduced signal of a video cassette recorder, bythe outputs of said carrier generator means and for providing a firstcolor difference signal and a second color difference signal; first combfilter means and second comb filter means for comb filtering said firstcolor difference signal and said second color difference signal,respectively and for producing respective outputs; first delay means andsecond delay means for delaying the outputs of said first comb filtermeans and said second comb filter means by a predetermined period,respectively; phase error detector means for detecting a residual phaseerror in the output of said first comb filter means and the output ofsaid second comb filter means during a gate pulse and producing anoutput; and phase error compensator means for compensating the residualphase error in the outputs of said first delay circuit and said seconddelay circuit by employing the output of said phase error detector.
 2. Adigital chrominance signal demodulation apparatus comprising: carriergenerator means for outputting two orthogonal carriers based on a firstinput signal and a second input signal;demodulator means fordemodulating a down converted carrier chrominance signal, which is areproduced signal of a video cassette recorder, by the outputs of saidcarrier generator means and for providing a first color differencesignal and a second color difference signal; first comb filter means andsecond comb filter means for comb filtering said first color differencesignal and said second color difference signal, respectively and forproducing respective outputs; first delay means and second delay meansfor delaying the outputs of said first comb filter means and said secondcomb filter means by a predetermined period, respectively; phase errordetector means for detecting a residual phase error in the output ofsaid first comb filter means and the output of said second comb filtermeans during a gate pulse and producing an output; and phase errorcompensator means for compensating the residual phase error in theoutputs of said first delay circuit and said second delay circuit byemploying the output of said phase error detector; said carriergenerator means includes an automatic frequency control circuit, a phaseshifter, a sin-table, and a cos-table; and wherein said carriergenerator means generates the two carriers such that they aresynchronized as a function of a modulation frequency of the downconverted carrier chrominance signal and a phase shift signal duringevery line.